1. Field of the Invention
The invention relates to a process for the preparation of N-alkylated anilines by reacting non-alkylated or only monoalkylated anilines with lower alcohols or the associated ethers at elevated temperature in the gas phase in the presence of an acid aluminum oxide catalyst.
Alkylated anilines are important industrial intermediates for the preparation of dyes, stabilizers, urethanes, ureas, pharmaceuticals and plant protection agents.
2. Description of the Related Art
It is known to prepare N-alkylated anilines by reacting anilines with alcohols either in the liquid phase or in the gas phase in the presence of acid catalysts. In the liquid phase process the reaction is carried out in the presence of liquid catalyst such as, for example, sulphuric acid, hydrochloric acid or phosphorus trichloride. These processes require a procedure under pressure. Because of the highly corrosive action of the acid catalysts the process must be carried out in special corrosion-resistant autoclaves. Nevertheless corrosion is not to be avoided. Additionally, the separation and disposal of the catalyst requires an additional expenditure.
In contrast, gas phase processes are carried out without pressure. Thus, according to the process described in DE-AS 10 31 796, a mixture of aniline vapour and alcohol vapour is passed through hot phosphoric acid at atmospheric pressure. In addition to corrosion problems, this process has the disadvantage that over a period of time the relatively large amount of phosphoric acid becomes unusable as catalyst and must be replaced by fresh phosphoric acid. The consumed catalyst must be sent for specialist disposal.
It is further known to carry out the alkylation of aromatic amines with alcohols on catalysts containing transition metals. U.S. Pat. No. 4,613,705 describes for example a vanadium/tin oxide contact catalyst. The conversion of aniline on this catalyst is however unsatisfactory and a high proportion of undesired ring alkylated products is found.
Houben-Weyl, Methoden der organischen Chemie (Methods of Organic Chemistry), volume XI/1 (1957), p. 116 discloses that aluminum oxides or silicates are also suitable catalysts for the alkylation of aromatic amines. However, as is further described, these catalysts have the disadvantage that their activity declines too rapidly and the life span is not sufficient for industrial purposes.
DE-AS 2,335,906 teaches that catalysts having a long service life are obtained if 0.1 to 20% by weight of phosphoric acid are added to silica. In order to avoid a rapid deactivation in this instance also and to ensure a long life span of the contact catalyst, it is necessary continuously to introduce phosphoric acid and/or alkyl phosphates during the alkylation; however, a portion of these phosphorus compounds is always discharged and must be separated from the reaction product.
Consequently the object was to make available catalysts with which different types of aniline compounds can be alkylated on the N atom in the gas phase and which do not possess the previously mentioned disadvantages. The catalysts must be distinguished by ready availability, long service lives and high activities and guarantee high conversion with good selectivity.